Existing X-Y positioners generally fall into two categories: (a) closed-loop servomechanisms; and (b) open-loop incremental positioners. Incremental positioners include basic designs that utilize the incremental step size inherent to the motor and more sophisticated implementations that divide the motor step into smaller parts through a technique called microstepping.
Servo systems typically provide higher speed and greater precision but are, in general, more complicated and expensive to produce.
Open-loop incremental positioners use step motors to drive the mechanical linkage. The motors are given pulses of steps. An upper limit exists on the pulse rate that a given motor and load combination will follow and this ultimately limits the achievable performance of the controller. A large step size allows increased velocity at the cost of resolution. This speed/resolution trade-off is characteristic of incremental positioners.
Step motors are, however, significantly less sensitive to friction in the mechanical linkage than are servo motors. Furthermore, they accurately position their shaft without the use of a position feedback element. As a result, incremental controllers are typically simpler and much less expensive to produce than servo controllers.
Some incremental positioner designs are beginning to use microstepping techniques to get around the speed/resolution trade-off. Problems are encountered, however, whenever step motors are used at other than their inherent step positions. Both the static position accuracy and dynamic torque characteristics vary from nominal values in between the designed step positions. This causes perturbations to occur when the motor is moving that may interact with the mechanical resonances and degrade the smoothness of the motion.
These characeristics of the motor impair the performance of X-Y positioners. The motor imperfections most important to X-Y positioner performance are:
(a) non-linearities in torque null positioning; and, PA1 (b) non-uniformity in the torque slope.
The first effect can obviously introduce error in the position of the mechanics both while moving and at rest. More importantly, it adds harmonics of the phase current frequency to the rotor motion. In addition, friction can pull the rotor appreciably away from the null torque position during constant velocity motion. When this happens, a nonuniform torque slope creates periodic disturbances in the rotor dynamic equilibrium. These disturbances contain the phase current fundamental frequency and its harmonics.
At specific values of the phase current frequency, the fundamental and harmonic frequencies generated by these imperfections can interact with the resonance in the rotor response. For example, an eight-pole, four-phase motor having a resonance at .omega..sub.o may experience significant excitation of the rotor resonance when the sinusoidal phase currents have frequencies of .omega..sub.o, .omega..sub.o /4, .omega..sub.o /8 and .omega..sub.o /2.